Yes, if by fasting it entails severe hardship or harm to yourself or the child if one is pregnant.
One should seek opinion of qualified Muslim medical professional; who will make risk assessment and advise whether there is possibility of you fasting with changes to medication routine or if there is no alternative and advises that there will be undue hardship and risk to your health by fasting then in such cases you will be excused.
For all missed fasts they have to be made up as soon as possible, ideally before the next Ramadhan but they can be made up at any time before death.
Individuals who have a chronic illness and are not expected to recover from that illness pay a compensation known as fidyah. The amount payable for each missed fast, scholars differ on the exact amount, but on average it's the price of 1.75kg - 2.2kg of wheat, probably £2-3 pound but one should check with scholar / local prices to confirm amount payable.
Anyone who does not have a chronic illness and who expects to recover from their illness, even before death, does not pay fidyah but must make up the missed fasts as soon as they recover from their illness.
Anyone who has a chronic illness and has paid fidyah and it so happens that they eventually recover from the illness to a stage that they are able to fast again then the fidyah paid will be considered optional charity (sadaqah) and the missed fasts must be made up.
Scholarly rulings on following links
http://www.askimam.org/public/question_detail/25001
http://seekershub.org/ans-blog/2010/09/12/brief-overview-of-expiatory-payments-fidya/
